A commonly held view in evolutionary biology is that speciation (the emergence of genetically distinct and reproductively incompatible subpopulations) is driven by external environmental constraints, such as localized barriers to dispersal or habitat-based variation in selection pressures. We have developed a spatially explicit model of a biological population to study the emergence of spatial and temporal patterns of genetic diversity in the absence of predetermined subpopulation boundaries. We propose a 2-D cellular automata model showing that an initially homogeneous population might spontaneously subdivide into reproductively incompatible species through sheer isolation-by-distance when the viability of offspring decreases as the genomes of parental gametes become increasingly different. This simple implementation of the Dobzhansky-Muller model provides the basis for assessing the process and completion of speciation, which is deemed to occur when there is complete postzygotic isolation between two subpopulations. The model shows an inherent tendency toward spatial self-organization, as has been the case with other spatially explicit models of evolution. A well-mixed version of the model exhibits a relatively stable and unimodal distribution of genetic differences as has been shown with previous models. A much more interesting pattern of temporal waves, however, emerges when the dispersal of individuals is limited to short distances. Each wave represents a subset of comparisons between members of emergent subpopulations diverging from one another, and a subset of these divergences proceeds to the point of speciation. The long-term persistence of diverging subpopulations is the essence of speciation in biological populations, so the rhythmic diversity waves that we have observed suggest an inherent disposition for a population experiencing isolation-by-distance to generate new species.
The conventional systems of veld management have a number of shortcomings, the main one being that the period of absence from grazing during the growing period is normally too short. This is due to the fact that a particular group of animals is allocated to a specific set of camps for the whole growing season. A more flexible system of veld management would allow the grazing procedure to be manipulated in such a way that the period of absence could be varied in relation to the rate of regrowth of the grass.The application of such a flexible system would involve the following: (a) the minimum number of camps required is calculated on the basis of at least four to five camps for each group of animals; (b) one quarter of the area of veld is rested; (c) the remaining camps are grazed whenever sufficient regrowth has taken place, and no group of animals is confined to particular camps or set of camps; (d) each camp is well grazed down and the number of camps grazed for the season will depend on weather conditions; during a dry year, more camps will be grazed and fewer camps during a wet year; (e) the camps grazed most selectively, are rested during the following season; and (f) burning of the veld is confined to those camps that have been rested during the previous year.
UITTREKSEL Die konvensionele stelsels van veldbeheer het 'n aantal tekortkominge die belangrikste waarvan is dat die periode van afwesigheid normaalweg te kort is gedurende die groeiperiode. Dit kan daaraan toegeskryf word dat 'n bepaalde groep diere op 'n bepaalde groep kampe vir die hele groeiseisoen gehou word.'n Meer buigbare stelsel van veldbeheer sou dit moontlik maak om die weiprosedure op so 'n manier te beheer dat die periode van afweisgheid gewissel kan word, na gelang van die mate van hergroei van die gras.Die toepassing van so 'n stelsel behels die volgende: (a) die minimum getal kampe word bereken op die grondslag van ten minste vier tot vyf kampe vir elke groep diere; (b) een kwart van die oppervlakte veld word gerus; (c) die oorblywende kampe word bewei sodra die gras voldoende hergroei het, en geen groep diere word tot 'n bepaalde stel kampe beperk nie; (d) elke kamp word goed kort bewei, en die getal kampe wat in enige seisoen bewei sal word, word deur weersomstandighede bepaal; in 'n droë jaar sal meer kampe en in 'n natjaar minder kampe bewei word; (e) die kampe wat selektief bewei is, word die volgende jaar gents en (f) die brand van veld word beperk tot die kampe wat die vorige jaar gems het.
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